I think its good that she has enough confidence in herself to wear something like that. Pity her mum didnt pick the right type of outfit for her. I wouldnt make her feel horrible by asking her to cover up but maybe have a word to her mum and see if she minded if her DD was a little more covered up if she comes over for a swim.

That being said, if it is making you all uncomfortable, offer her a pair of your shorts. Tell her you dont want her getting sunburnt or something.